How can I use the kernel function in my C code?

First, i want to apologize for my English, it's not my mother language but i will do my best.

I'm a new developer in Debian, before that I only do cross-plateform code. Since cross-plateform isn't in my constraint anymore, I want to be as close as possible from the kernel or the binary (like ls, mount and other binary in C).

What bug me is I can't find the source code for the binary in my debian 7.1.0 and 8.0.1. I know this is a common question, but it's seem that all I do finish by a failure (and be new in Debian doesn't really help me :/ )

For example : i want to see the code of ifup / ifdown command. I search and i see that ifupdown isn't a binary but a script so I can directly view inside, but in my Debian it's a ELF binary.

Does a documentation to get a source code for a binary Debian exist ? I'm really frustrated because I have the feeling that the answer is just under my nose ! It drive me crazy :/

The second thing is, for example, i want to use AES for crypt file. AES exist in the kernel and I prefer to use the AES kernel rather than another AES (I simply trust more the kernel source code). Is there a way to use Kernel function in user space ?

I hope I doesn't make a mistake with my question. Thank you for your reading, stack overflow help me a dozen and dozen time in the past, so thank you.